MRI Physics 2 — Questions and Answers
Question 1: Which property of hydrogen makes it the most commonly used nucleus in clinical MRI?
- High natural abundance and large magnetic moment (Correct answer)
- Low atomic mass and zero spin
- Absence of electron cloud interference
- High atomic number and radioactivity
Correct answer: High natural abundance and large magnetic moment
Hydrogen (¹H) is abundant in the body and has a large gyromagnetic ratio, producing strong MRI signals.
Question 2: What is the purpose of the radiofrequency (RF) pulse in an MRI sequence?
- To tip net magnetization away from B0 (Correct answer)
- To increase static magnetic field strength
- To suppress gradient coil noise
- To align hydrogen nuclei with each other
Correct answer: To tip net magnetization away from B0
The RF pulse at the Larmor frequency tips the net magnetization vector away from the longitudinal axis to induce signal.
Question 3: T2* relaxation differs from T2 relaxation primarily because T2* includes:
- Magnetic field inhomogeneities in addition to spin-spin interactions (Correct answer)
- Only spin-lattice interactions
- Chemical shift effects only
- Gradient-induced dephasing only
Correct answer: Magnetic field inhomogeneities in addition to spin-spin interactions
T2* accounts for both intrinsic spin-spin relaxation (T2) and additional dephasing caused by local magnetic field inhomogeneities.
Question 4: In a gradient echo sequence, what replaces the 180° refocusing pulse used in spin echo?
- A reversed readout gradient (Correct answer)
- A second 90° RF pulse
- A slice-select gradient reversal
- A phase-encoding rewinder gradient
Correct answer: A reversed readout gradient
Gradient echo sequences use a reversed readout gradient to rephase spins, making them sensitive to T2* rather than T2.
Question 5: What determines the slice thickness in a standard 2D MRI acquisition?
- The bandwidth of the RF pulse and the slope of the slice-select gradient (Correct answer)
- The repetition time and echo time
- The number of phase-encoding steps
- The field of view and matrix size
Correct answer: The bandwidth of the RF pulse and the slope of the slice-select gradient
Slice thickness = RF bandwidth / (gyromagnetic ratio × gradient strength); a wider RF bandwidth or shallower gradient yields a thicker slice.
Question 6: Which artifact arises when signal from outside the FOV wraps around to the opposite side of the image?
- Aliasing (wrap-around) artifact (Correct answer)
- Gibbs ringing artifact
- Chemical shift artifact
- Susceptibility artifact
Correct answer: Aliasing (wrap-around) artifact
Aliasing occurs in the phase-encode direction when anatomy outside the FOV is undersampled, causing it to appear on the opposite side.
Question 7: The signal-to-noise ratio (SNR) in MRI is proportional to which of the following?
- Voxel volume × √(NEX) × √(scan time factors) (Correct answer)
- 1 / (TR × TE)
- Field of view / matrix size only
- Flip angle / repetition time
Correct answer: Voxel volume × √(NEX) × √(scan time factors)
SNR increases with larger voxel volume, more signal averages (NEX), and longer acquisition times that allow more k-space sampling.
MRI Physics 2
Which property of hydrogen makes it the most commonly used nucleus in clinical MRI?